1. Field of the Invention
The present invention relates to a DC-DC converter that boosts a direct-current input voltage and outputs a resultant voltage.
2. Description of the Related Art
A charge pump boosting circuit that uses a capacitor and diodes is commonly used as a DC-DC converter for which an input power supply voltage is low and having a relatively small current-carrying capacity. Japanese Unexamined Patent Application Publication No. 10-323046 discloses a power supply circuit including a rectangular wave generation circuit and a circuit for performing voltage doubling rectification upon a rectangular wave output from the rectangular wave generation circuit using a capacitor and diodes.
A charge pump boosting circuit generates a voltage of a predetermined amplitude and smooths the voltage to generate a direct-current voltage in principle. The charge pump boosting circuit superimposes an alternating voltage (a square wave that is a switching waveform in normal cases) on the direct-current voltage and smooths a resultant voltage to generate a voltage of a double amplitude value.
A charge pump boosting circuit that uses a capacitor and diodes can boost a voltage with a simple circuit. However, the following problems arise.
Since voltage boosting is performed with only a capacitor and diodes, it is impossible to increase a charging/discharging current for the capacitor and respond to the demand for a large load current.
Since a circuit configuration is provided in which, after a direct-current voltage has been generated by smoothing, an alternating voltage is superimposed on the direct-current voltage, two diodes are always needed for voltage doubling rectification. Large losses therefore occur at the diodes.
Since a large pulsed current flows from the capacitor into a switching element, a large loss occurs at the switching element. The amount of heat generated in the switching element is therefore large and it is difficult to increase power conversion efficiency. In order to reduce a pulsed current, it is necessary to reduce the capacitance of the capacitor. In this case, however, it is difficult to output a large current.